Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in publications by C. H. Bennett et al entitled “Experimental Quantum Cryptography” and by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992).
The above mentioned publications each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons, and Bob randomly measures the polarization or phase of the photons. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized to within a few tens of nanoseconds during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. During the QKD process, Alice uses a random number generator (RNG) to polarization-encode or phase-encode a photon and Bob uses another RNG to randomly select a polarization-basis or phase-basis, as appropriate, to measure Alice's sent photon.
One of the major problems in quantum cryptography is the generation of random numbers to randomly encode the photons. It is desirable that the RNG used be fast and truly random, in contrast to pseudo-randomness used in computer-based RNG's.
For commercial quantum cryptographic systems, the RNG must be simple and cost effective. Further, the underlying physical processes and any associated electronic circuits must be easily analyzed and understood. From this point of view, current optical quantum random number generators are by no means the best possible.
There are a number of prior art RNGs, including some based on single photon detection. However, the latter tend to be expensive and relatively slow (˜100 kHz) for the purposes of modern data transmission systems. Likewise, there are currently RNGs based upon noise quantization. These are fast, easy to reproduce and are relatively cheap. However, the problem is in the noise source. Most of the noise sources are based upon a particular type of semiconductor noise, such as Zener diode noise, transistor noise etc. These types of noise are complex in nature, and the mathematical models needed to describe them require a large number of parameters and are not particularly accurate. Further, the effect of aging of semiconductor components on the random generation process is difficult to plausibly test, especially for high-speed output devices.
What is needed therefore is a fast, truly random and relatively simple true random number generator (TRNG) for high-performance systems, such as QKD systems.
The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.